As is known, in hydraulically actuated automatic transmissions, the hydraulic control of transmission shift elements such as hydraulically actuated clutches is effected by means of pressure regulation valves.
One possible principle for pressure regulation is the so-termed pilot control principle. In this, a pilot control valve acting as an electric pressure control valve converts the electric signal from an electronic transmission control unit into a pressure different from (lower than) the system pressure. The pressure signal of the pilot control valve, the so-termed pilot control pressure, passes via a pilot control line to a pressure regulation valve, by means of which the pilot control pressure is then amplified to a higher pressure signal so that the shift element is supplied with a working pressure and volume flow appropriate for its filling.
The pressure regulation (modulation) of the pilot control valve designed for example as a pressure reduction valve takes place by virtue of a hydraulic slider, which adjusts its working pressure in that the slider modulates between a high feed pressure and a low tank pressure.
If a transmission shift element is in the non-actuated condition, i.e. it is not acted upon by a working pressure and is connected to a tank or transmission housing at the pressure of the surroundings, the transmission shift elements and the lines leading to them can drain, or air can be precipitated from the working medium. This results in a long filling time and hence a long shifting time of the clutch. Furthermore the regulating quality of the working pressure is poor, since it shows large pressure fluctuations. The precipitation of air produces considerable scatter and interactions, which the transmission software can only insufficiently picture in the form of a model or take into account in controlling the shift elements.
DE 10 2007 020 346A1 shows a hydraulic control system for this type, in which the collection of air in a non-actuated piston space of a transmission shifting element is avoided in that the piston space is connected to a fluid reservoir.
To prevent air precipitation and draining of the transmission shift elements, in the non-actuated condition the pressure in a transmission shift element is set at a low pressure potential. This pressure potential, which is only slightly higher than the ambient pressure of the transmission, will be called the pre-filling pressure in what follows. By pre-filling the clutch lines, air precipitation and therefore the variance of controllability are considerably reduced, so that according to the present state of the art hydraulically actuated clutch systems have clutch pre-filling means. Such a transmission control system is known from DE 102 39 915A1.
The advantage of lines pressurized with the pilot control pressure, henceforth referred to as pilot control lines, generally consists in their arrangement within the hydraulic control system. By virtue of the fitting position of the hydraulic control unit in the oil sump, draining of the pilot control lines is thus excluded owing to gravity. There are only air precipitations, which are trapped in the new condition during the fitting of the hydraulic control system but which no longer occur once operation of the transmission has started. However, in the case of hydraulic controls not located in the oil sump of the transmission, the problem arises that an at least partial draining of the pilot control lines takes place, particularly when the transmission is static and the transmission pump is not delivering any pressure.
In addition it has disadvantageously been found that when the pilot control valve has not been actuated for a long time, air is even precipitated in the pilot control lines when the oil pump is operating, this mainly occurring only in lines that carry a large oil volume.
This results in poor regulation of the pilot control pressure when the pressure regulation valve is actuated, leading to similar or even more pronounced negative effects on the adjustment of the clutch pressure.